Last month, when a 300-foot-deep sinkhole opened up at a phosphate plant in Mulberry, draining acidic waste into the aquifer below, the owner, fertilizer industry giant Mosaic, alerted the Florida Department of Environmental Protection, the U.S. Environmental Protection Agency and Polk County.

But neither Mosaic nor the three governmental agencies alerted the nearby homeowners who draw their water from the same aquifer. They didn't learn about the 215 million gallons of contaminated water that fell into the aquifer until the incident became public on Friday, three weeks later.

Why the secrecy?

A DEP spokeswoman said state law doesn't require the state or the company involved to notify anyone until there's some sign the pollution has migrated outside the property where it went into the aquifer.

"Should there be any indication of offsite migration of contaminated groundwater, rules require the notification of affected parties," the DEP's Dee Ann Miller said in an email to the Tampa Bay Times. "However, to date there is no evidence of offsite movement or threat to offsite groundwater supplies."

That's different from what happened when another large sinkhole opened at that same phosphate plant site in 1994. Plant employees found the sinkhole on Monday, June 27, and the news about it hit the papers less than a week later, on Friday, July 1.

In between then and now, the Legislature changed the rules. Miller pointed to a 2005 law that says any business discovering contamination has spread beyond its boundaries has to notify the DEP no later than 10 days after the discovery, and then the DEP has 30 days to notify the neighbors.

The law says nothing about warning anyone before the contamination spreads through the aquifer.

While that may be legal, it's wrong to keep silent in such situations, contends Bradley Marshall, senior associate attorney in the Tallahassee office of Earthjustice, a legal firm that has frequently done battle with phosphate miners.

"There is still a moral obligation to let people know what happened to our drinking water supply," he said. "There is a duty (by government) to let people know what is going on."

Mosaic announced that it would provide free well tests and bottled water to anyone who's nervous about the state of his or her well. Miller, the DEP spokeswoman, said her agency was coordinating with the company in that effort, noting it was "above and beyond" state requirements.

So far, 70 of the plant's neighbors have asked for the tests and water, according to the company.

"Understandably, some of our neighbors who live near the New Wales facility are concerned about water coming from their wells," Mosaic spokeswoman Eileen Stuart said Monday. "So, to allay any concern whatsoever, we are offering to have their wells tested by a third party and that process began today. Additionally, we are providing bottled water to any neighbors who request it, even though our testing continues to show that this water has not moved offsite."

She did not respond to questions about why Mosaic finally decided to reveal the sinkhole last week.

The aquifer — a lens of fresh water floating underground in limestone caverns — is the source of much of Florida's drinking supply. Contamination of that aquifer poses a risk of cutting off the drinking water for millions of people.

The aquifer is also the source of water for processing phosphate at Mosaic's plant. Mosaic has a permit from the Southwest Florida Water Management District to pump 69.6 million gallons of water a day from the aquifer for the facilities on its 330,000 acres spread across Central Florida. Some of it — no one knows how much — is used to dilute pollution created by the mining and manufacturing process before it's dumped back into area rivers and creeks.

Sometimes, overpumping the aquifer can cause sinkholes. In 2010, when an 11-day cold snap prompted strawberry farmers to pump nearly 1 billion gallons water a day from the aquifer to protect their crops, more than 85 new sinkholes opened up across eastern Hillsborough County.

However, a spokeswoman for the water agency, commonly known as Swiftmud, says overpumping is only one of several potential causes.

"There are lots of things that can contribute to the development of a sinkhole, including land construction activities, heavy rainfall, groundwater withdrawals and fluctuations in aquifer levels," Swiftmud spokeswoman Susanna Martinez Tarokh said. "We cannot speculate on what could have caused this incident."

Companies have been mining phosphate from Florida's landscape since the 1800s. The factories that turn the phosphate into fertilizer churn out a radioactive byproduct called phosphogypsum. To dispose of it, the phosphate industry stacks it up into white sandy mountains.

Meanwhile, when the plant uses water it comes out of the factory as corrosive as battery acid. First the water goes into cooling ponds, where some evaporates. Then it's pumped to the top of the stacks. Rainfall adds millions of gallons more.

Mosaic workers became aware of the sinkhole when water levels in a pond atop a 120-foot gypsum stack dropped 2 feet between readings on Aug. 27. They began diverting water from the pond, which can hold up to about 250 million gallons.

The pond drained out, with acidic water laced with sulfate and sodium falling into the sinkhole along with an unknown amount of gypsum, a fertilizer byproduct with low levels of radiation.

Mosaic must drop some equipment like sonar into the sinkhole to map its dimensions and meanwhile is using a recovery well that pumps 3,500 gallons of water per minute from the aquifer back to the surface to try to recapture all the contaminants.

However, more contaminated water will leak with every new rainfall until the sinkhole is filled, something company officials say is likely to take months.

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